[1] Prior to Gecko 8.0 (Firefox 8.0 / Thunderbird 8.0 / SeaMonkey 2.5), Gecko sent the WebSocket close event to the listener as a simple event. Support for CloseEvent was implemented in Gecko 8.0.

[2] Prior to Gecko 12.0 (Firefox 12.0 / Thunderbird 12.0 / SeaMonkey 2.9), Gecko reported the close code CLOSE_NORMAL when the channel was closed due to an unexpected error, or if it was closed due to an error condition that the specification doesn't cover. Now CLOSE_GOING_AWAY is reported instead.